We were able to connect to the MySQL database server (which means your username and password are okay) but not able to select the database.
The MySQL error was: Access denied for user 'kardec_ac_admin'@'%.rz.rwth-aachen.de' to database 'kardec_aachen'.
Currently, the database is kardec_aachen. The username is kardec_ac_admin and the database server is dbserver-2.rz.rwth-aachen.de.
For more help, see the Installation and upgrading handbook. If you are unsure what these terms mean you should probably contact your hosting provider.